Traditional mechanical relay devices receive a control signal at a coil element and the coil element responds by producing a magnetic field for bringing together contact elements of the relay. The contact elements connect to nodes of a controlled circuit. A control circuit driving the coil element thereby determines conductivity, i.e., performs a switching function, between nodes of the controlled circuit. Such mechanical relays have suffered from relatively slow switching times and the need for a relatively large coil drive current to accomplish switching. Solid state relays address these concerns by providing high speed switching with relatively low drive current requirements. Accordingly, solid state relays are now a preferred form of control in many control applications.
An important function of any relay is isolation between the control circuit and the controlled circuit. The relay provides a switching function between nodes of the controlled circuit, but no electrical connection exists between the control circuit and the controlled circuit. Thus, the control circuit may be a low power digital logic circuit with the relay device acting as a control interface to a much higher power circuit. For example, the control circuit could be a computer-based lighting control panel and the controlled circuit a high powered theater lighting system. A set of solid state relays interconnecting the control panel and the lighting system provide programmed control of the theater lighting system. An important aspect of the relationship between the control panel, operating at relatively low voltage and our rent levels, and the lighting system, operating at much higher voltages, is the electrical isolation provided by the solid state relays. If the high voltages or currents in the lighting system were introduced into the control panel, extensive damage to the control panel is likely.
When a solid state relay receives excess currents, i.e., beyond specified ratings such as in a dead short situation, internal failures of the solid state relay can cause a short between the line voltage terminals, i.e., those connected to high voltage high current devices, and the input control terminals, i.e., those connected to low voltage low current devices. If this happens the line voltage can feed into the normally isolated control terminals to reach and destroy the control circuit.